This invention relates to aromatic polyglycidyl polyether resins useful in coating and electrodeposition compositions.
The majority of commercially-available aromatic polyglycidyl polyether resins are prepared by reacting a polyhydric phenol such as 2,2-bis(4-hydroxyphenyl)propane (BPA) with epichlorohydrin. This glycidation reaction, when BPA is the polyhydric phenol, normally proceeds to produce resins having schematic formula I: ##STR1## in which G stands for glycidyl and BA stands for the group resulting from the removal of both hydroxyl groups from the polyhydric aromatic compound BPA. The average value of n can be varied within the range of from 0 to 18. For example, in the commercial epoxy resins EPIKOTE.RTM. 828, 1001, 3003, 1007 and 1009, typical average values of n are 0.1, 2, 4, and 12, respectively. It follows from the above formula that the number of BA units and secondary hydroxyl groups in the resin molecule equals (n+1) and n, respectively.
The reactivity of the glycidyl group can be exploited to produce modified resins. For example, by etherification with aliphatic diols such as hexanediol or with aliphatic triols such as trimethylolpropane, resins may be obtained of the schematic formulae II, III and IV: EQU EP-HD-EP (II) EQU EP-TMP-EP (III) EQU TMP-(EP).sub.3 (IV)
in which HD stands for the hexanediol moiety, TMP for the trimethylolpropane moiety and EP for the resin moiety represented by formula (I) above, in which now one glycidyl group has been converted into a bridging group "--O--CH.sub.2 --CHOH--CH.sub.2 --". It will be clear that the etherification reactions yielding products (II) and (III) have resulted in the introduction of two additional secondary hydroxyl groups into the resin molecule, a marked increase of the molecular weight without a substantial change in the number of epoxy groups per molecule, and (by definition) a marked reduction of the epoxy group content (EGC). In product (IV), three additional secondary hydroxyl groups have been introduced. Furthermore, since in the product (III) only two of the three methylol groups of TMP have reacted with a glycidyl group, the third methylol group is left unchanged and it follows that in product (III) also a primary hydroxyl group has been introduced into the resin molecule.
An important application of polyglycidyl polyether resins is in the field of surface coatings such as electrodeposition coatings. In cathodic electrodeposition, improved flow and lower viscosities of resin or binder molecules are very desirable, as better flow allows a smoother surface in film coatings and lower viscosity allows high solids contents in the final binders. The viscosity must be low enough to allow the formulation of a paint; however, an organic solvent provides some of the flow during stoving. A better inherent flow of the binder would therefore permit a lower solvent content. Low viscosity combined with improved flow could eventually lead to solventless binders and related paints.
Flow in cathodic electrodeposition binders may be provided by incorporation of long aliphatic chains, e.g., fatty acids or aliphatic monoamines. Both routes lead to a strongly reduced number of glycidyl groups per molecule, however, which, as the molecular weight is increased, is accompanied by an even more marked reduction of the EGC.
It is therefore an object of the invention to provide low-viscosity resins without substantially reducing the number of glycidyl groups per molecule. In one embodiment, the invention seeks to solve the problem of introducing glycidylester moieties of C.sub.5-18 branched aliphatic acids by selective reaction with primary or secondary hydroxyl groups in polyglycidyl polyether molecules.